The invention relates to a waveguide and particularly to a waveguide having a structure comprising a connection of a photonic-crystal line-defect-waveguide to a total reflection confinement waveguide, for enhancing the optical coupling efficiency.
A photonic crystal is an artificial optical crystal of which the permittivity has been periodically and greatly modulated. The photonic crystal generally has an angular frequency zone called a xe2x80x9cphotonic band gap (PBG)xe2x80x9d in which light cannot propagate in any direction within the photonic crystal.
FIG. 1 shows an example of the structure of a two-dimensional photonic crystal.
This two-dimensional triangular-lattice photonic-crystalline material 3 has a structure such that photonic crystal holes 2 having a very small diameter are arranged in a triangular lattice form in a high-permittivity medium 1. A semiconductor, which in many cases has a specific permittivity of about 12, is generally used as the high-permittivity medium 1. In the case of the two-dimensional triangular-lattice photonic-crystalline material 3, guidelines about a combination of the pitch (or lattice constant) of the holes with the hole diameter, which can provide PBG, are disclosed, for example, in J. D. Joannopoulos, R. D. Meade and J. N. Winn, xe2x80x9cPhotonic Crystals, Modeling the Flow of Light,xe2x80x9d Princeton University Press, pp. 125-126. For example, in the case where circular holes of a photonic crystal are arranged in a triangular lattice form on a dielectric substrate having a permittivity of 11.4. PBG occurs regardless of the direction of vibration of electric field and the direction in which the holes elongate, when the following requirements are met: (r/a)=0.48; and {(xcfx89xc3x97a)/(2xcfx80xc3x97c)}=about 0.5, wherein r represents the radius of holes, a represents the pitch of circular holes of the photonic crystal, xcfx89 represents the angular frequency of light, and c represents the speed of light in vacuum.
FIG. 2 shows an example of a photonic-crystal line-defect-waveguide which has been prepared using the two-dimensional triangular-lattice photonic-crystalline material 3 shown in FIG. 1.
This photonic-crystal line-defect-waveguide 5 has a structure such that two two-dimensional triangular-lattice photonic-crystalline materials 3 of the type shown in FIG. 1 are disposed close to each other. A large number of holes 2 are arranged in a triangular lattice form in a high-permittivity medium 1. One row of non-hole portion, that is, a line defect portion 4, is provided in a middle portion between the large number of holes 2. In this case, light of angular frequency within PBG cannot propagate through the fully crystallized portion other than the line defect portion 4, but can propagate through the line defect portion 4. That is, the line defect portion 4 functions as a waveguide.
The photonic-crystal line-defect-waveguide 5 shown in FIG. 2 can lead light and, in addition, as a result of a reflection of the properties of the photonic crystal, has other properties such as small group velocity or wavelength dispersion. Therefore, the formation of an optical device utilizing the above properties is considered. When attention is directed only to properties as a waveguide for leading light, however, the photonic-crystal line-defect-waveguide 5 is sometimes inferior to the total reflection confinement waveguide.
FIG. 3 shows a construction of a total reflection confinement waveguide. As with an optical fiber, the total reflection confinement waveguide 10 can lead light, which has been introduced into one end, to other end. The total reflection confinement waveguide 10 is a dielectric line having a smooth surface, has a simple structure, and undergoes a less structural fluctuation than the photonic-crystal line-defect-waveguide 5 shown in FIG. 2 and, thus, is easy to prepare a form as designed. Therefore, a waveguide having a lower loss than the photonic-crystal line-defect-waveguide 5 can be easily prepared. Since, however, properties such as small group velocity and wavelength dispersion. in the total reflection confinement waveguide 10 are not significant as compared with those possessed by the photonic-crystal line-defect-waveguide 5, the connection of the photonic-crystal line-defect-waveguide 5 to the channel waveguide is considered effective for utilizing mutual advantages of the total reflection confinement waveguide 10 and the photonic-crystal line-defect-waveguide 5.
FIG. 4 shows a conventional waveguide. The construction of this waveguide is such that the total reflection confinement waveguide shown in FIG. 3 is connected to the photonic-crystal line-defect-waveguide shown in FIG. 2.
The end face of the total reflection confinement waveguide 10 as a channel waveguide is connected to the end face 11 of the line defect portion 4 in the photonic-crystal line-defect-waveguide 5 having the structure shown in FIG. 2. The end face 11 of the line defect portion 4 is flat in a portion close to the line defect portion, including the line defect portion 4 and the portion other than the line defect portion 4. The angle of the connections 14, 15 of the surfaces 12, 13 of the total reflection confinement waveguide 10 to the end face 11 of the photonic-crystal line-defect-waveguide 5 is 90 degrees. The total reflection confinement waveguide 10 is formed of a high-permittivity medium 16 which is identical to the high-permittivity medium 1 of the photonic-crystal line-defect-waveguide 5.
Since the total reflection confinement waveguide 10 is formed of the high-permittivity medium 16, the confinement of light in the total reflection confinement waveguide 10 is significant and the width of the lateral distribution (profile) of light, which propagates through the total reflection confinement waveguide 10, is substantially equal to the width of the total reflection confinement waveguide 10. A part of light, which propagates through the photonic-crystal line-defect-waveguide 5, however, is spread from the line defect portion 4 to the horizontal hole row. In order to match the profile of light, which propagates through the total reflection confinement waveguide 10, with the profile of the spread light, the width of the total reflection confinement waveguide 10 is made larger than that of the photonic-crystal line-defect-waveguide 5.
According to the conventional waveguide, however, in the connection between the total reflection confinement waveguide 10 and the photonic-crystal line-defect-waveguide 5, since the width of the total reflection confinement waveguide 10 is larger than that of the photonic-crystal line-defect-waveguide 5, the portion around the renter of the total reflection confinement waveguide 10 is kept continuous with the portion around the center (around the line defect portion 4) of the photonic-crystal line-defect-waveguide 5. On the other hand, in the peripheral portion apart from the center of the total reflection confinement waveguide 10, since a uniform structure on the total reflection confinement waveguide 10 side is connected to a periodic arrangement structure of holes on the photonic-crystal line-defect-waveguide 5 side, the structure of the peripheral becomes discontinuous. The photonic crystal per se does not permit the transmission of light with angular frequency within PBG, and, thus, the electromagnetic field energy of light distributed outside the width of the line defect portion 4 in the total reflection confinement waveguide 10 is disadvantageously reflected from a portion around the end face 11 of the photonic-crystal line-defect-waveguide 5 and thus cannot enter the photonic-crystal line-defect-waveguide 5. For this reason, a combination of the total reflection confinement waveguide 10 with the photonic-crystal line-defect-waveguide 5 has a problem that, despite the fact that they are close to each other in the profile of the propagation light, the optical coupling efficiency is poor.
Accordingly, it is an object of the invention to provide a waveguide which can realize a connection of a total reflection confinement waveguide to a photonic-crystal line-defect-waveguide with high optical coupling efficiency.
According to the first feature of the invention, a waveguide comprises: a photonic-crystal line-defect-waveguide comprising a high-permittivity medium, holes arranged in two groups of holes in a predetermined pattern within the high-permittivity medium, and a hole-free line defect portion provided at the interface of the two groups; and a total reflection confinement waveguide which has a width identical to or substantially identical to the width of the line defect portion in the photonic-crystal line-defect-waveguide and is connected to the end face of the line defect portion.
According to this construction, the width of a line defect portion provided on the photonic-crystal line-defect-waveguide is identical to or substantially identical to the width of the total reflection confinement waveguide connected to the line defect portion. By virtue of this construction, coupling loss attributable to reflection in the connection between the photonic-crystal line-defect-waveguide and the total reflection confinement waveguide can be reduced, and the optical coupling efficiency can be enhanced.
According to the second feature of the invention, a waveguide comprises: a photonic-crystal line-defect-waveguide comprising a high-permittivity medium, holes arranged in two groups of holes in a predetermined pattern within the high-permittivity medium, and a hole-free line defect portion provided at the interface of the two groups, one row of holes among the holes arranged along one side face of the line defect portion having been cut into a semicircular form; and a total reflection confinement waveguide which has a width identical to or substantially identical to the width of the line defect portion in the photonic-crystal line-defect-waveguide and is connected to the end face of the line defect portion.
According to this construction, the width of a line defect portion provided on the photonic-crystal line-defect-waveguide is identical to or substantially identical to the width of the total reflection confinement waveguide connected to the line defect portion. By virtue of this construction, coupling loss attributable to reflection in the connection between the photonic-crystal line-defect-waveguide and the total reflection confinement waveguide can be reduced, and the optical coupling efficiency can be enhanced. Further, in the photonic-crystal line-defect-waveguide, one row of the photonic crystal along the side face, to which the total reflection confinement waveguide is connected, is cut into a semicircle. By virtue of this, the end face of the line defect portion does not spread in the widthwise direction, and loss attributable to leakage onto the photonic crystal surface can be reduced.
According to the third feature of the invention, a waveguide comprises: a total reflection confinement waveguide having predetermined width and length; and a photonic-crystal line-defect-waveguide comprising a high-permittivity medium, holes arranged in two groups of holes in a predetermined pattern within the high-permittivity medium, and a hole-free line defect portion, with a width identical to or substantially identical to the width of the total reflection confinement waveguide, provided at the interface of the two groups, one end of the total reflection confinement waveguide having been connected to the line defect portion, the high-permittivity medium having been projected toward both sides of the total reflection confinement waveguide to form a protective portion.
According to this construction, the width of a line defect portion provided on the photonic-crystal line-defect-waveguide is identical to or substantially identical to the width of the total reflection confinement waveguide connected to the line defect portion. By virtue of this construction, coupling loss attributable to reflection in the connection between the photonic-crystal line-defect-waveguide and the total reflection confinement waveguide can be reduced, and the optical coupling efficiency can be enhanced. Further, the formation of a protective portion so as to embed both sides of the total reflection confinement waveguide can reduce the optical coupling loss and, at the same time, can increase the mechanical strength of the photonic crystal near the connection between the photonic-crystal line-defect-waveguide and the total reflection confinement waveguide.